The combination of decreasing feature sizes and increasing chip sizes is leading to a communication crisis in the area of VLSI circuits and systems. It is generally realized that the exponential growth of semiconductor chip capabilities cannot continue indefinitely, and that fundamental limits exist. These limits arise not from difficulties associated with the reduction of gate areas and delays, but rather from the difficulties associated with the interconnections as dimensions are scaled downward and chip areas continue to increase. It is anticipated that the speeds of semiconductor circuits will soon be limited by the interconnection delays, rather than gate delays. Furthermore, the trend of increasing bit-rate introduces problems in conductor design, as the conductors must be treated as transmission lines for high frequencies. This problem has been designated as the communication bottleneck, and is affecting wide-bandwidth switching systems and high-throughput computer system architectures.
Improvements in manufacturing processes and material handling have allowed a constant reduction of feature sizes in integrated circuits. Additionally the number of transistors in application designs has increased steadily. As further chip integration and complexity is obtained, the chip I/O demand increases. Such high I/O densities become difficult with current bonding techniques such as solder bump bonding.
As the number of components and connections of systems increase, the difficulty in assembling and maintaining these systems increases too. Systems become impractical or excessively expensive to build. One problem encountered is the bandwidth required to move data between subsystems. Sufficient bandwidth cannot be supported by state of the art interconnection technology. One example is the lack of sufficient bandwidth on backplanes to support large broadband switching systems. One approach is to demultiplex high bandwidth signals to multiple lower bandwidth signals. However, if connections are to be maintained at their original rate, the designer is faced with maintaining the transmission line integrity from printed circuit board to printed circuit board. This is normally achieved by designing a backplane, circuit cards and connectors that are transmission lines. However, there is a limit to the number of connections that can be made this may.
Another disadvantage of electrical interconnection technology is excessive crosstalk. Any current flowing in a conductor induces a magnetic field. The result of this magnetic field is inductive coupling, and electrons in adjacent conductors will travel with this field and set currents circulating in the conductors. Capacitive coupling between striplines also causes crosstalk. To alleviate this problem, the electrical interconnection must be set at a distance large enough to prevent this signal from having any effect on system performance. This creates fundamental rules for circuit routing. Furthermore, electrical interconnect paths must reside near a ground plane to ensure that stray electric fields are properly terminated.
Electrical interconnections suffer from an additional problem of sensitivity to external electromagnetic interference. The fields that reside in the vicinity of the interconnection lines induce currents in them, causing erroneous signals in the lesser case to circuit damage in the worst case. Designers must shield integrated circuits by using conducting envelopes to prevent any field from entering the shielded volume. Special interconnection line design is required for lines that connect any shielded circuits. Shielding also worsens the problem of heat removal in the circuit, as ventilation is restricted.
Optical interconnections, rather than electrical, offer a solution to the problems plaguing conventional electrical interconnections. For example, optical interconnections offer a freedom from mutual coupling effects (i.e., cross talk) not afforded by conventional electronic interconnects. In addition, they offer increased bandwidths and immunity to electro-magnetic interference. This potential advantage of optics becomes more important as the bit rate increases, as the strength of mutual coupling associated with electrical interconnects is proportional to the frequency of the signals propagating on the interconnect lines.
Many telecommunications applications require the capability to switch any signal in an input array of N signals to one output signal in an array of N output signals. Telecommunications switching, transport and routing systems make widespread use of networks called multistage interconnection networks (MIN), to accomplish this function. These are alternating layers of fixed interconnection patterns and arrays of basic switching modules for two signals, called bypass-exchange switches. The bypass-exchange switch is an elementary switch for two signals that may either pass the two input signals unaffected or interchange them. Each layer of interconnection links and switch arrays is defined as a stage. MINs are capable of performing dynamic interconnections between a source point and a target point, by varying the settings of the switches. An example of a MIN network is the Omega network.
MINs have been proposed and utilized for computer architecture and telephone switch gear where the signals are electronic. The one dimensional network has a planar configuration (two dimensions) that suits electronic signals, Optics, however, can propagate in free space, thus easily allowing for a more efficient three dimensional topology. Planes of switching elements are interconnected by optical beams, exploiting the spatial bandwidth available in the optical domain. The switching element in this case will have four signals at the input and the output.
Traditional approaches to designing the elementary bypass-exchange switch for optical signals, include building a hybrid opto-electronic semiconductor circuit employing photodetectors, electronic circuitry for switching and lasers or optical modulators. Optical signals are converted to electric signals, which after amplification and electronic switching, are used to drive the lasers. The disadvantages of this process include low efficiency and reduced signal to noise ratios due to noise factors introduced and confinement to sub-gigabit modulation rates.
An alternative approach is an all optical switch which performs the switching function without converting the signals from the optical domain to the electrical domain and back. One option is to use polarization based switching that first combines two signals when they are polarized linearly and perpendicularly to each other. The switching itself is done by exchanging polarization states of the two signals. Combining and splitting the two perpendicularly polarized signals has been demonstrated with polarized beam splitters and also recently demonstrated with a birefringent computer generated hologram. Disadvantages of this approach are the bulkiness of the required optical devices and the difficulty of size reducing a system constructed in this fashion.